The present invention relates to methods of manufacturing a color display tube having a magnetic quadrupole post-focusing mask formed by a plate of a magnetizable material having rows of apertures provided though oppositely facing sides of the plate, which plate magnetized so that, cyclically, a north pole, a south pole, a north pole and a south pole are formed along the circumference of each aperture.
The invention also relates to devices for carrying out such methods.
Such a method of manufacturing a color display tube having a magnetic quadrupole post-focusing mask is disclosed in U.S. Pat. No. 4,135,111 to Verweel issued Jan. 16, 1979. The object of magnetic post-focusing is to increase the transmission of the mask. In tubes without post-focusing, a very large part, for example 80 to 85%, of the electrons is intercepted by the so-called shadow mask. By using magnetic post-focusing, the apertures in the mask can be enlarged because, as a result of the focusing in the apertures, the electron spots on the screen are considerably smaller than the apertures so that sufficient space is still present between the electron spots of the various electron beams to avoid their overlapping onto adjacent phosphors.
In known tubes, the mask is formed by a magnetizable plate which has a large number of apertures and which is magnetized so that, cyclically, a north pole, a south pole, a north pole and a south pole are present along the circumference of each aperture. The plate may be manufactured from a ferromagnetic material or frm a non-ferromagnetic material on which a layer of magnetizable material has been provided. A magnetic quadrupole lens is present in each of the apertures. Such a lens focuses the electron beam in one direction and defocuses it in a direction at right angles thereto. Because the magnetic field is perpendicular to the electron beam, quadrupole lenses are comparatively very strong so that a comparatively small magnetization will suffice.
The magnetization of such a mask in the above-mentioned U.S. Pat. No. 4,135,111 is carried out by means of one or more writing heads each having four pole shoes comprising coils. The pole shoes are magnetically connected by a yoke. If an electric current flows through the coils in the correct direction, cyclically, a north pole, a south pole, a north pole and a south pole are formed along the circumference of each aperture. The writing head is constructed so that the pole shoes are placed in the facing corners of four adjacent apertures. A result of this is that four poles, two north poles and two south poles, are formed at a small distance from each other. This formation weakens the post-focusing action of the apertures. Oppositely directed poles of adjacent apertures should, therefore, be provided as far remote from each other as possible.
Moreover, in the known method, the longitudinal direction of the rows of apertures is the same as the longitudinal direction of the phosphor lines on the display screen. Further, the poles are present at the corners of each aperture. This means that the longitudinal direction of the linear spot formed by the quadrupole lens, which spot direction should be the same as the longitudnal direction of the phosphor lines, is situated in the longitudinal direction of the rows of apertures. The distance between two phosphor lines luminescing in the same color is, in this case, equal to the pitch between the rows of apertures.
The distance between the phosphor lines and, hence, the number of phosphor lines on the display screen can be increased by causing the longitudinal direction of the rows of apertures to vary at an angle of about 45.degree. with the longitudinal direction of the phosphor lines and causing the longitudinal direction of the linear spot to vary at an angle of approximately 45.degree. with the longitudinal direction of the rows of apertures, i.e., in the longitudinal direction of the phosphor lines on the display screen.